1. Field of the Invention
The present invention relates to a process for preparing a curable aqueous polymer dispersion, to the polymer dispersion obtainable by this process, to a coating composition comprising at least one such polymer dispersion, and to a process for producing a coated substrate using such a polymer dispersion.
2. Description of the Background
Radiation-curable coating materials, based for example on polyester and polyether acrylates and methacrylates, have acquired great industrial significance and found broad application. Owing to their crosslinkability, they are suitable in particular for preparing radiation-curable coating compositions, e.g., paint formulations, which can be cured rapidly by actinic radiation. They are employed to coat both nonabsorbent substrates, such as metals, plastics, and glass, and absorbent substrates, such as wood, paper, and leather.
In order to prevent environmental pollution through solvent emissions and to satisfy corresponding legal provisions, there is increasing use of radiation-curable paint formulations based on polymer dispersions in which the water content of the dispersion medium is as high as possible. Additional requirements are imposed on coating compositions for outdoor applications. For example, they are intended to have good stability in the case of weathering effects, and in particular with respect to UV radiation. At present, urethane acrylate dispersions are used substantially for outdoor applications. A disadvantage of these dispersions is their high production cost, putting them at an economic disadvantage.
In the preparation of coating compositions it is known to use polymers based on vinyl monomers which have a relatively narrow molecular weight distribution (polydispersities Mw/Mn<5.0) and relatively low molecular weights (Mn between 1000 and 10 000).
EP-A-0 554 783 describes a process for continuous polymerization of vinyl monomers which forms vinyl polymers having a number-average molecular weight Mn in the range from 1000 to 20 000 and a polydispersity Mw/Mn<3.5, the polymerization being conducted in an annular gap reactor. These vinyl polymers contain no incorporated ethylenically unsaturated double bonds and can therefore not be crosslinked free-radically by what is known as self-crosslinking. In general they are also unsuitable for the preparation of polymer dispersions based on dispersion media that contain water.
EP-A-0 650 978 describes free-radically crosslinkable copolymers having a number-average molecular weight in the range from 1500 to 6000 and a polydispersity of from 1 to 4.0 which are obtainable by polymer-analogously reacting a vinyl polymer with a compound by means of which ethylenically unsaturated double bonds are introduced into the polymer. EP-A-0 650 979 describes a process for preparing such free-radically crosslinkable copolymers where the polymer-analogous reaction takes place in an extruder. Because of their molecular weight, these copolymers are highly viscous or solid. They are therefore suitable, preferably, for use as binders for powder coating. There is in fact a general description of formulations of these copolymers in the form of solutions and dispersions. For use in stable, water-containing dispersions, however, these copolymers are still in need of improvement.